This invention relates to an amplifier circuit arrangement having first and second stages in cascade, the first stage comprising a long tail pair of first and second transistors of the same conductivity type and first and second resistors coupling output electrodes of said first and second transistors respectively to a first voltage reference point. The second stage comprises a current amplifier circuit and a third resistor, the output electrode end of said first resistor being coupled to a second voltage reference point via said third resistor and the input current path of said current amplifier circuit in that order, and the output electrode end of said second resistor being coupled to said second voltage reference point via the output current path of said current amplifier circuit.
Long tail pairs of transistors are often employed in amplifier circuit arrangements, particularly when the arrangements are in integrated circuit form. Individual integrated circuit manufacturing techniques tend to favour the fabrication of transistors of a particular conductivity type rather than transistors of the opposite conductivity type, for example, because those of the particular conductivity type take the form of so-called "vertical" transistor structures whereas those of the opposite conductivity type take the form of so-called "lateral" transistors. "Vertical" transistor structures tend to have higher gains and cut-off frequencies than "lateral" transistor structures, with the result that in many applications it is usual to try to avoid employing "lateral"transistors if at all possible. However, sometimes other considerations come into play which militate against this. One of these is a requirement in some applications that noise superimposed on the supply voltage of the arrangement must not, or must only negligibly, become manifest in the output signal of the arrangement. An amplifier arrangement of the general kind specified in the first paragraph is known from U.S. Pat. No. 4,710,728. In this known amplifier the (differential) output of the first stage is constituted by the output electrode ends of the first and second resistors. Signal voltages occurring at these points are, of course, relative to the first voltage reference point because they are set up across the first and second resistors respectively. Thus, if a said signal voltage were applied to the control electrode of a third transistor the common electrode of which is connected to the first voltage reference point, any noise occurring at the first voltage reference point will not have any effect on the signal occurring between said control and common electrodes. However, this would mean in practice that, unless the provision, for example, of more than two supply potentials could be tolerated, the third transistor would have to be of the opposite conductivity type to said same conductivity type, which as stated above is often unsatisfactory. It is an object of the invention to enable this problem to be mitigated.